A noncontact type tonometer, in which intraocular pressure of the patient's eye is measured by directing compressed air to the cornea of the patient's eye and applanating thereof, is well-known. U.S. Pat. No. 3,585,849, for example, discloses a noncontact tonometer in which the intraocular pressure of the patient's eye is measured based on an elapsed time interval during which a surface of the cornea deforms into a flat state from a convex state while being deformed by a compressed air pulse directed thereto. In the noncontact type tonometer mentioned above, the pressure of the air pulse necessary to deform the cornea to the flat state is indirectly obtained according to the time interval elapsed during which the surface of the cornea deforms into the flat state from the convex state, assuming that the pressure of the air pulse is controlled correctly based on the optimum control condition.
Additionally, in Japanese Patent Application after substantive examination, Laid-open No. SHO 63-58577, a noncontact type tonometer is disclosed in which the pressure of the air pulse corresponding to the flat state of the cornea is directly obtained from a pressure sensor arranged in a device for producing the air pulse.
However, in the former tonometer, measuring accuracy thereof depends on whether or not the air pulse producing device is controlled under the predetermined optimum control condition. Therefore, the measuring accuracy of the air pulse pressure in the tonometer will not be reliable unless the air pulse producing device is controlled under the predetermined optimum control condition.
The determining method of the air pulse pressure according to the above tonometer will be described based on FIG. 5. FIG. 5 is a graph to explain the determining method of the air pulse pressure in the conventional tonometer, in which the ordinate shows the air pulse pressure, the abscissa shows time, f is a standard curvature which shows change of the air pulse pressure versus time (abbreviated "standard p-t curvature" hereinafter), g is an actually measured curvature with various errors which shows change of the air pulse pressure versus time (abbreviated "actual p-t curvature" hereinafter), h is a curvature which shows change of light quantity reflected from the cornea and maximum light quantity is obtained at a time t because the cornea deforms to the flat state at the timet, p(t) is the air pulse pressure obtained from the standard p-t curvature f at the time t, t' is a time at which the maximum light quantity will be obtained if the air pulse pressure is measured without any error and p'(t) is the air pulse pressure obtained from the standard p-t curvature f at the time t'.
According to FIG. 5, the air pulse pressure is measured based on a condition (which deviates from a predetermined optimum condition) including various errors, such as atmospheric changes around the tonometer and mechanical error produced in piston mechanism of the air pulse producing devices. The air pulse pressure is determined as p(t) at the time t according to the standard p-t curvature f in FIG. 5, since the air pulse pressure is obtained based on the time t at which the maximum light quantity reflected from the cornea is detected due to its flat state. Thereafter, the intraocular pressure of the patient's eye is calculated from the air pulse pressure p(t). Such obtained air pulse pressure p(t) deviates from the air pulse pressure p'(t) to be obtained without any error at the time t'. As mentioned above, a defect in the measuring error of the air pulse pressure will be caused by the above various errors which exist in the conventional tonometer.
In the latter tonometer, the measuring error of the air pulse pressure is not caused by the atmospheric change or the mechanical error mentioned above, and reproducibility in measuring of the air pulse pressure is good because the intraocular pressure of the patient's eye is calculated based on the air pulse pressure directly detected by the pressure sensor at the time when the maximum light quantity reflected-from the cornea is obtained. However, in the latter tonometer, after the air pulse pressure is produced, the change of the air pulse pressure versus time is not monitored, therefore, early detection of the abnormal state in the air pulse pressure caused by a malfunction of the piston in the air pulse pressure producing device or binding in a nozzle part formed in the air pulse pressure producing device to direct the compressed air pulse to the cornea of the patient's eye cannot be conducted. Further, excessive air pulse pressure may be directed to the cornea of the patient's eye, giving discomfort to the patient, since the change in the air pulse pressure versus time is not monitored in the latter tonometer.